This invention relates to a pipe and particularly, although not exclusively, relates to a pipe per se and methods of producing a pipe. Embodiments aim to extend the lifetime and/or reduce the risk of premature failure of a pipe which may carry a high pressure fluid (e.g. liquid (optionally containing particulates), gas or a mixture of the two) and/or be subjected to high external forces, in use. Preferred embodiments relate to pipes for use in the oil and/or gas industries for example flexible risers arranged to transport fluids between floating production units and a sub-sea wellhead. Other preferred embodiments may be used in the aerospace industry, in process industries, in geometrical and mining industries and in industry in general.
It is well known to produce pipes by extruding molten polymers through a suitably shaped die and water quenching the extruded pipe, for example by directing it into a cool water bath and/or by spraying cool water on its outside. However, for fast crystallizing polymers and/or polymers having a relatively high glass transition temperature such methods may produce pipes which have significant differences in crystallinity from the outside to the inside of the pipe wall. For example, the outside wall of the pipe may have an amorphous (or very low crystallinity) skin due to rapid quenching of an extruded melt used in making the pipe, whereas on moving inwards, the crystallinity of the pipe wall may increase significantly. Such differences in crystallinity across the pipe wall lead to residual hoop stress. For example, in a 4.2 inch (10.6 cm) outside diameter (OD) pipe, the residual hoop stress (calculated as described hereinafter) may be in excess of 5.6 MPa. In general, residual stress may be caused by molecular orientation frozen in during the manufacturing process or thermal stresses due to different cooling rates. Such residual stress is a consequence of the thermal contraction of the melt being restricted during non-uniform solidification which freezes in a strain. Such high residual stress can lead to problems. For example, if such pipes are sawn, the pipe may shatter as the stresses are released. Alternatively, and/or additionally, the pipes may be more susceptible to failure through fatigue and/or stress in use. A highly stressed pipe is more likely to fail (e.g. crack catastrophically) in a shorter time in use than a pipe with lower residual stress.
It is desirable to maximise the useful lifetimes of pipes and/or reduce the risk of premature failure. It is an object of the present invention to address this problem.